OMAPDSS: OMAPFB: check for matching memory size early

If the size of memory region that is being set up is the same as before,
we don't have to do memory and layer busy checks.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>